The field of the invention is industrial control systems such as process controllers and programmable controllers, and particularly, communication systems for connecting such controllers together in a network.
Historically, industrial control communications networks have been organized into star, multidrop, or ring configurations. In a star configuration, one controller forms the center and acts as the network master. Separate lines extend from this master controller to all the other "slave" controllers. A multidrop network such as that disclosed in U.S. Pat. No. 4,149,144 also includes a single master controller which connects to a single data trunk. The slave controllers are connected to the data trunk by lines which are "dropped" from the trunk and communications is established between the master controller and a selected slave controller by a "polling" process. In such a multidrop network communications between slave controllers is possible only through the master controller. With a ring configuration each controller is linked to two other controllers in a loop arrangement. Messages are relayed along from controller to controller and a network master acts to delete any messages that have travelled full circle and have not been acknowledged by a controller in the ring.
A problem with any of these prior communications networks is that the entire system fails if a malfunction occurs in the master controller. In the star configuration the slave controllers become isolated from one another when the master controller fails in the multidrop system all communications stop because the polling process cannot occur. In the ring network any controller failure inhibits communications. As a result, it is common practice to employ redundancy as a means of preventing complete network failure in these prior systems and such a solution is expensive.
Another solution is to employ a contention scheme on a multidrop network. In such a system there is no master controller, but instead, each controller "contends" for access to the data link. When access is obtained, a message may be sent directly to any other controller in the network. Numerous contention techniques are known to the art and although they do eliminate the dependence on a master controller for maintaining communications, no single contention technique is universally applicable to all traffic patterns. Also, special hardware must be added to detect the "collision" of messages on the data link when such techniques are used and this adds to the cost and complexity of the network.